Chemical milling of titanium,refractory metals and their alloys

ABSTRACT

A PROCESS OF CHEMICALLY MILLING TITANIUM, REFRACTORY METALS, AND THEIR ALLOYS, RESULTING IN DECREASED ABSORPTION OF HYDROGEN, THUS ALLOWING CHEMICAL MILLING OF BETA TITANIUM AND ALPHA BETA TITANIUM, WHICH NORMALLY CANNOT BE CHEMICALLY MILLED IN CONVENTIONAL CHEMICAL MILLING SOLUTIONS BECAUSE OF HYDROGEN EMBRITTLEMENT. ANOTHER ADVANTAGE OF THIS PROCESS IS A BETTER CHEMICALLY MILLED SUR FACE FINISH AND MORE RAPID AND UNIFORM METAL REMOVAL RATES THAN OBTAINABLE IN CONVENTIONAL REFRACTORY METAL CHEMICAL MILLING SOLUTIONS. THE PROCESS ALSO IS SUITABLE FOR ALPHA TITANIUM ALLOYS. THE CHEMICAL MILLING SOLUTIONS ITSELF CONSISTS OF A MIXTURE OF NITRIC ACID, HYDROFLUORIC ACID OR FLUORIDE SALTS AND DERIVATIVES OF CARBONIC ACID SUCH AS CARBAMIDE, IN CONCENTRATIONS OF AT LEAST 10 GRAMS PER LITER OF FINAL SOLUTION.

United States Patent Ofice 3,788,914 Patented Jan. 29, 1974 3,788,914 CHEMICAL MILLING OF TITANIUM, REFRAC- TORY METALS AND THEIR ALLOYS John Gumbelevicius, St. Louis County, Mo., assignor to McDonnell Douglas Corporation, St. Louis, Mo. No Drawing. Continuation-impart of abandoned application Ser. No. 661,753, Aug. 3, 1967. This application Nov. 18, 1971, Ser. No. 200,143

Int. Cl. C23f I/ U.S. Cl. 156-18 Claims ABSTRACT OF THE DISCLOSURE A process of chemically milling titanium, refractory metals, and their alloys, resulting in decreased absorption of hydrogen, thus allowing chemical milling of beta titanium and alpha beta titanium, which normally cannot be chemically milled in conventional chemical milling solutions because of hydrogen embrittlement. Another advantage of this process is a better chemically milled surface finish and more rapid and uniform metal removal rates than obtainable in conventional refractory metal chemical milling solutions. The process also is suitable for alpha titanium alloys. The chemical milling solution itself consists of a mixture of nitric acid, hydrofluoric acid or fluoride salts, and derivatives of carbonic acid such as carbamide, in concentrations of at least 10 grams per liter of final solution.

REFERENCE TO PRIOR APPLICATION This application is a continuation-in-part of prior copending application Ser. No. 661,753 filed Aug. 3, 1967, now abandoned, entitled Chemical Milling of Titanium, Refractory Metals and Their Alloys.

BACKGROUND OF THE INVENTION Presently it is very difiicult to chemically mill titanium and refractory metals other than the alpha alloys of titanium, because of the high affinity of beta and alpha beta alloys for hydrogen. High hydrogen absorption results in great internal stresses which tend to crack the metal. It is very difiicult to machine beta and certain alpha beta titanium alloys and consequently these alloys have not been fully utilized in the aerospace industry, although the alloys do exhibit very attractive mechanical properties. It has always been the desire of the aero-space industry that a method of chemically milling these alloys be found. Additionally, the present invention provides a very convenient method of pickling, cleaning, and surface preparation for such alloys.

SUMMARY OF THE INVENTION The present invention comprises a chemical milling solution for milling titanium, refractory metals, and their alloys, specifically beta and alpha beta titanium alloys, with little hydrogen absorption in the alloys and uniform titanium removal rates. The solution comprises nitric acid, hydrofluoric acid, and derivatives of carbonic acid in concentrations of at least 10 grams per liter of solution. In addition, small percentages of surfactants are added when the smoothness of the chemically milled surface is important.

DETAILED DESCRIPTION OF THE INVENTION Example No. I

The following is a specific detailed disclosure of a preferred process of chemically milling titanium, specifically beta titanium alloy.

(1) The specimen (beta titanium alloy) is cleaned with trichloroethylene to remove contaminants such as grease, oil, etc. This pre-treatment is necessary for removal of oil, grease and other contaminants normally found on parts in metal fabrication. The purpose of this cleaning is twofold: (a) to eliminate contamination of chem-milling solution, and (b) to provide a clean surface for proper maskant adhesion. Both factors are important to produce acceptable parts; however, these factors do not affect hydrogen absorption and thus are incidental with respect to the invention.

(2) The specimenn is dipped into a neoprene base maskant. This is a proprietory compound supplied by Turco Products, Inc. The maskant is at room temperature, and has a viscosity of 40 seconds as measured with Zahn No. 5 viscosimeter. This is a conventional solution and the step is well known in this art.

(3) The coated specimen is permitted to dry at room temperature (about 76 C.) until tack free condition. This takes about 20 minutes.

(4) Dipping and drying between dipping is repeated three more times to apply four coats of mask which provides about 0.016 in. thick coating.

(5) After the final coat, the specimen is kept at room temperature for about six hours to permit evaporation of solvents from the coating.

(6) Then the masked specimen is baked at 200 F. for one hour. This treatment polymerizes the coating and renders it inert to chemical milling solution.

(7) The chem-milling template, previously made from steel, is applied to the specimen. The template is designed to serve as a guide for the scribing knife to obtain the chem-milling pattern. The knife used is a commercial type X-Acto scribing knife.

(8) After all the lines for chem-mill pattern are cut through the maskant, but not into the metal, the template is removed and the maskant is peeled by hand from the areas to be chem-milled.

(9) The specimen is immersed into an activating or cleaning solution for one minute. The solution consists of 30% by volume of 67.18 wt. percent nitric acid, 2% by volume of wt. percent hydrofluoric acid and the remainder water. The solution is kept at room temperature. This step is necessary to remove the film formed on the refractory metal where the maskant is removed. If the film is not removed, the etching solution will not etch uniformly.

10) The specimen is transferred into the chem-milling solution. The solution composition is as follows: 682 g. of nitric acid, 63 g. of hydrofluoric acid, 50 g. of carbamide, 0.01 g. dodecyl benzene sulfamic acid, and water to make a total volume of one liter. The solution is at F. and is agitated by means of an electric stirrer. The specimen is suspended in the solution using plastic coated wire and is left in the chem-mill solution for 20 minutes.

(11) After 20 minutes, the specimen is removed from the chem-milling solution, rinsed with cold water, and the amount of metal removal is measured using a micrometer. 0.024 in. of metal was removed which is a metal removal rate of 0.0012 in. per minute.

(12) As the desired metal removal is 0.030 inch (half the thickness of the test specimens), the test specimen is immersed again into the same activating solution for one minute and returned to the chem-milling solution for an additional five minutes, to obtain the desired metal removal.

(13) The specimen is removed, rinsed with cold water and the metal removal is checked again with a micrometer to ascertain that 0.030 in. of metal have been removed.

(14) The maskant from areas which had not been chem-milled is removed by hand peeling.

The foregoing process can be used to chem-mill other titanium alloys and refractory metals and their alloys. The following alloys are considered as refractory: columbium, molybdenum, tungsten, and tantalum.

Chemical milling of refractory metals is of major importance since there are no known chem-milling processes for some of these metals, such as tantalum and tungsten. Furthermore, the known processes for columbium present hydrogen absorption problems which are eliminated by the use of the present process.

The chem-milling solution used in the foregoing Example No. I contains 0.1 gram of dodecyl benzene sulfamic acid. This ingredient or some other surfactant, such as alkyl benzene sulfonate, is necessary in the present process except when the smoothness of the final chemically milled surface is not important.

The surfactant amount is normally small and the range can vary from 0.001% to about 2%. The maximum amount is not critical as excess surfactant does not produce any harmful effect upon chemical milling. The minimum amount is determined by how much the solution surface tension is reduced. The reduction of surface tension is not critical in general except for extremely accurate work.

The chemically milling solution contains preferably 70% by volume of commercially available 67.8 wt. percent nitric acid. The range of nitric acid concentration can be varied between about 126 and about 682 grams of pure nitric acid per liter of final solution. The use of nitric acid is critical.

Between 1 and 20% by volume of commercially available hydrofluoric acid can be used, although 4-8% of said hydrofluoric acid is preferred. These ranges can be expressed as 12.6251.6 grams of 70% hydrofluoric acid or about 8.8 to about 176.1 grams of pure hydrofluoric acid per liter of final solution. While hydrofluoric acid is preferred, fluoride salts such as lithium fluoride, ammonium fluoride, sodium fluoride, potassium fluoride, cobalt fluoride, and the like, may be used.

Carbamide is the preferred carbonic acid derivative and is present in amounts of at least grams per liter of final solution. Preferably the range is from about 20 to about 200 grams per liter. At 200 grams per liter a super saturated solution is produced and increased concentration of carbamide is not more effective.

However, increasing the concentration to 300 or 500 grams per liter is not detrimental to the final process. Other carbonic acid derivatives which may be used include urea nitrate, urea oxalate, semicarbazide, and mixtures thereof and the like.

The temperature range of the chemical milling process may be between about 30 F. and 230 F., although the preferred range is about 80 to about 190 F.

The areas of the titanium which are not to be chemically milled may be coated with certain inert coatings, such as neoprene, high melting wax, or other materials inert to the chemical milling solution. Such materials are commercially available.

Hydrog en content of the milled article is measured in parts of hydrogen per million parts of final product. Aeronautical Materials Specifications maximum permissible hydrogen concentrations are 150-200 parts per million, depending upon the titanium alloy involved. These figures are conservative and for less critical applications than aero-space vehicles higher hydrogen contents could be tolerated, although above about 500 parts per million is not generally satisfactory.

During the development of this process, Ti-13V-11Cr- 3A1 titanium alloy was used because this alloy has beta crystalline structure and is more susceptible to hydrogen embrittlement than other alloys. When chem-milled in standard solution, this alloy absorbed about 4,000 parts per million hydrogen by weight. This amount of hydrogen produced sutficient stresses to cause the test piece to crack immediately after chem-milling. A similar piece chemmilled in the present solution contained about parts per million of hydrogen by weight. This does not affect physical and mechanical properties of titanium and is within limits established by Aeronautical Materials Specifications.

Another term used in the industry in describing the effects of hydrogen absorption is hydrogen embrittlement, which is based upon the degree of deterioration of mechanical and physical properties of titanium as hydrogen is absorbed. Generally this means cracking and weakening of the structure.

The time of chemical milling varies widely depending on the amount of metal removal. Normally the rate of metal removal is about 0.001 inch per minute per surface. Therefore, if the desired amount of metal removal is 0.030 inch, the treatment time would be 30 minutes. The majority of present aero-space parts require metal removal of about A; of an inch which would take about two hours in chemical milling time. On the other hand, if the only purpose for chemical milling is to obtain a clean surface, 0.001 inch removal is sufficient and the treatment time Would be about one minute.

The use of the carbamide results in suppressing the hydrogen absorption of the titanium and additionally results in a very substantial increase in the metal removal rate. A solution having the same concentration of in gredients but without carbamide has a very low metal removal rate, about one-tenth of the rate obtained when using carbamide. Accordingly, this very beneficial secondary effect of the carbamide is desirable and very useful in many operations.

The reaction mechanism of titanium milling with the composition of this invention also has the advantage of eliminating emission of noxious and dangerous gases. The gases emitted by this reaction are nitrogen and carbon dioxide both of which are normally present in the atmosphere. Presently known chemical milling operations release nitrogen oxide, hydrogen, or chlorine which requires elaborate and expensive equipment to remove from the effluent air stream.

The invention is intended to cover all changes and modifications of the examples of the invention herein chosen for purposes of the disclosure, which do not constitute departures from the spirit and scope of the invention.

What is claimed is:

1. A method of chemically milling and surface treating a material selected from the group consisting of titanium, columbium, molybdenum, tungsten, tantalum and their alloys without excessive hydrogen absorption comprising the steps of:

(a) exposing said material to a solution containing per liter of solution about 126 to about 682 g. of nitric acid, an amount of a fluoride producing material sufficient to provide the equivalent of about 8.8 to about 176.1 g. of pure hydrofluoric acid and a derivative of carbonic acid selected from the group consisting of carbamide, urea nitrate, urea oxalate and semi-carbazide in an amount equivalent to at least about 10 g. of carbamide,

(b) at a temperature of about 30230 F.,

(c) for a period of time suflicient to remove the undedesired material. 1

2. The method of claim 1 wherein the material is titanium alloy.

3. The method of claim 2 wherein the titanium is beta and mixed beta alpha alloys.

4. The method of claim 3 including the step of adding from about 0.001% to about 2% of a surfactant to the solution.

5. The method of claim 1 wherein the fluoride ion producing, agent is hydrofluoric acid and the carbonic acid derivative is carbamide.

6. The method of claim 1 wherein the chemical milling rate is on the order of 0.001 inch/minute/surface 5 and the hydrogen absorption is below 200 parts/million. 7. The method of claim 6 wherein the material is beta titanium alloy.

8. The method of claim 1 wherein the solution includes from about 0.001 to about 2.0% of a surfactant. 5

acid derivative is carbamide and the solution includes 10 from about 0.001% to about 2.0% surfactant.

References Cited UNITED STATES PATENTS 2,177,751 10/1939 Sikorski 252-79.4 3,108,919 10/1963 Bowman et al 156-18 WILLIAM A. POWELL, Primary Examiner US. Cl. X.R. 252-79.4 

